This invention generally relates to the gasification of coal with an inert heat transfer medium (IHTM). More particularly, this invention is concerned with more efficient reheating of the IHTM with increased solid-to-solid contact between the IHTM and hot char.
Coal may be gasified using steam and hot IHTM solids. The particular details of the coal gasification reaction form no part of this invention and a detailed description of this reaction is not required.
The coal gasification reaction produces a mixture of ash, unreacted char and IHTM. All or a part of char is burned to provide heat for reheating the IHTM. The reheated IHTM is cycled back to the coal gasifier. In general, two systems for burning the char and reheating the IHTM have been proposed. In one system, the char and ash are separated from the IHTM and then the char is burned to produce a hot gas. This gas is then used to reheat the IHTM. The rate of heat transfer between the hot gas and the lack of uniform heating make this system inefficient unless multiple beds are used.
In the second system, the char, ash and IHTM are fed to a fluidized combustor or lift pipe where the unreacted char is burned. The dilute-phase fluidized system combustor for burning the unreacted char and reheating the IHTM has a number of shortcomings. For example, the particle residence is short and there is very little contact between the solids. The heat from burning the char is transferred to the gas and then to the IHTM solids. Computer modelling of this poor heat transfer mechanism and the short residence time indicates that the IHTM solids will likely be several hundred degrees below that of the char particles and that the temperature of the burning char particles can, therefore, exceed the ash fushion temperature. Fusion of the ash in the char could cause agglomeration of the char and cause plugging, slugging and fouling. Fluidized IHTM reheating systems are naturally hydrodynamically unstable and sensitive to gas rates, gas-to-solids ratios and particle sizes. Fluidizing a mixture of ash, char and IHTM also causes equipment erosion problems.